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The Crucial Role Of Non-thermal Electrons In Solar Flare-induced Dimming Of He I 10830

Presentation #303.07 in the session “Solar Physics Division (SPD): Flares II”.

Published onJun 18, 2021
The Crucial Role Of Non-thermal Electrons In Solar Flare-induced Dimming Of He I 10830

While solar flares are predominantly characterised by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of orthohelium spectral transitions (He I 10830Å and the He I D3 lines near 5876A) have shown evidence of such dimming in some weak flares, usually followed by enhanced emission. It has been suggested that the presence of non-thermal collisional ionisation of helium by the electron beam, followed by recombinations to orthohelium, is responsible for overpopulating the orthohelium levels leading to stronger absorption. However it has not been possible observationally to preclude the possibility of overpopulating orthohelium via enhanced photoionisation of He I by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of non-thermal electron beam-driven flares where (1) both non-thermal collisional ionisation of Helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam, and a range of non-thermal electron beam low-energy cutoff values, were simulated. For each simulation the He I 10830A line was forward modelled. In order to obtain flare-induced dimming of the He I 10830A line it was necessary for non-thermal collisional ionisations to be present. Further, the effect was more prominent in flares with harder non-thermal electron spectrum (larger low-energy cutoff values) and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of orthohelium line emission as a diagnostic of flare energy transport.


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